Digital coherent receivers generally use an adaptive equalizer to compensate for differential group delay (DGD) on the transmission paths. In a 40 Gbps or 100 Gbps system, the rotating state of polarization changes due to a change in the external conditions such as vibration or temperature of the transmission path.
The converging state of an adaptive equalizer varies depending on the polarization state at the time of initial pull-in (or startup), and the adaptive equalizer may have an undesirable converging state that causes degradation of signal quality. Depending on the state of convergence at the startup, the DGD tolerance of the adaptive equalizer may fall and the resultant signal quality (such as bit error rate: BER) may deteriorate.
An adaptive equalizer by itself cannot determine if it resides in the undesirable converging state causing degradation of signal quality. In general, suitability of the converging state cannot be determined unless the BER is monitored, and it takes time for the entire system to start up.
One approach for improving the DGD tolerance of an adaptive equalizer is to increase the number of taps of the finite impulse response (FIR) filter. However, in reality, it is impossible to increase the number of taps without limitation because of the limited circuit scale. It is desirable to correct the undesirable converging state (or positional deviation of tap coefficients) and provide stable performance of equalization without increasing the number of taps.
A method of bringing the weighting center of the tap coefficients toward the tap center of an adaptive equalizer is known. See, for example, Japanese laid-open Patent Publication No. 2012-119923. This method is used to reduce the remainder of equalization.